Cold Cathode Fluorescent Lamps (CCFL) are extensively used today as backlights for computer liquid crystal displays (LCD) and LCD televisions. CCFL lamps are typically driven in a push pull configuration such that current flows alternatively in each direction to ensure long life and uniform operation. A CCFL driver thus must provide a pair of complementary drive signals which are out of phase with each other. Typically, a CCFL driver requires a pair of driving signals which are 180° out of phase with each other, however this is not meant to be limiting in any way and 90° phase difference is also required in some particular applications.
Systems that use a plurality of CCFL lamps for a single display require synchronization between the CCFL drivers so as to avoid interference caused by non-synchronized operation. Further synchronization is often required with a system display signal, such as a video scanning signal, associated with a horizontal scanning frequency, so as to avoid a water wave effect. Such synchronization is required for single and multiple CCFL driver systems.
U.S. Pat. No. 5,615,093 issued Mar. 25, 1997 to Nalbant, the entire contents of which is incorporated herein by reference, is addressed to a power converter circuit for driving a CCFL lamp comprising a controller circuitry. The controller circuitry includes an oscillator capable of self synchronizing to the resonant frequency of the components, and exhibits a sawtooth timing circuit for the oscillator. The clock frequency driving the CCFL is half the frequency of the sawtooth timing circuit. Thus, in order to synchronize a plurality of CCFL controller circuits of the type described by Nalbant, a clock signal double the driving frequency must be propagated or a frequency doubler circuit provided thereby allowing for synchronization from the CCFL driving output.
U.S. Pat. No. 5,923,129 issued Jul. 13, 1999 to Henry, the entire contents of which is incorporated herein by reference, is addressed to a power conversion circuit driving a CCFL. The clock oscillating circuit operates at twice the frequency of the direct drive network and provides the fundamental frequency of operation. Thus, in order to synchronize a plurality of CCFL controller circuits of the type described by Henry, a clock signal double the driving frequency must be propagated, or a frequency doubler circuit provided, thereby allowing for synchronization from the CCFL driving output.
In order to synchronize a plurality of CCFL controller circuits among themselves and/or with a video horizontal scanning frequency, a plurality of connections is typically required including a synchronization signal, a phase signal, and a connection indicating if the CCFL controller circuit is a master or slave. If the operating frequency of the CCFL controller circuit is to be equal to the video horizontal scanning frequency, a frequency doubler is also required since, as described above, the clock is typically double the operating frequency.
These additional circuitry and pins will inevitably increase the complexity and cost of the controller IC or the inverter control circuit.